The present invention relates to a cured silicone rubber article having a protective coating layer of a urethane resin-based coating composition or, more particularly, to a cured silicone rubber article, of which the surface layer is of the silicone rubber as such or a silicone resin-based coating layer, having a protective coating layer of a urethane resin-based coating composition with very firm adhesive bonding to the substrate surface to be resistive against abrasive rubbing as well as to a method for the preparation of such an article. In particular, the invention is applicable to a silicone rubber-made push button switch covering member of which the key top surface bearing an indicia to indicate the function of the particular key top which should be highly resistant against wearing under repeated pushing operations with a finger tip.
As is well known, various kinds of modern electric and electronic instruments, such as telephones, pocketable calculators, office-automation instruments, computers and the like, have a push button switch panel containing a plurality of push button switches. Sometimes, such a push button switches are desired to be visible or recognizable even in a dark place as in various kinds of instruments born on a transportation machine such as air crafts, automobiles, boats and the like by proving an internal illuminating means.
A push button switch panel currently under practical applications basically consists of a base board, on which a plurality of pairs of fixed contact points are provided, and a push button switch covering member made from a rubbery material having elasticity and having a plurality of push button units corresponding to the pairs of fixed contact points on the base board on which the push button switch covering member is mounted. A single push button unit in the push button switch covering member consists, in principle, of a base plate, a key top plate and a riser part connecting the base plate and the key top plate to form a dome-like protrusion on the base plate. It is usual that the key top plate is provided on the upper surface with an indicia to indicate the function of the particular switch while the key top plate is provided on the lower surface with a movable contact point which comes into contact with the fixed contact points on the base board either directly or indirectly through a click plate and the like when the key top plate is pushed down with a finger tip of the operator so as to close the electric circuit. When the key top plate is released from the pushing-down force, it regains the undepressed disposition by means of the elastic resilience of the riser part to open the electric circuit.
By virtue of the simple structure requiring a small number of parts and easiness of assemblage as compared with traditional mechanical switches, the push button switch panels of the above described type are widely used in various fields mentioned above. Among various kinds of rubbery materials, silicone rubbers are the most widely used rubbery materials for the push button switch covering member, which is an integral body prepared by molding and curing a silicone rubber, in respects of the unique and advantageous properties thereof such as the precision moldability, excellent electric insulation, high heat and cold resistance, weatherability, resistance against fatigue by repeated bending, elastic resilience and so on.
Conventional silicone rubber-made push button switch covering members, however, have several problems and disadvantages that, as an inherency of silicone rubbers, the top surface of the key top plate gives a feeling of tackiness to the operator's finger tips to disturb smooth key operation resulting in a decrease in the operating efficiency, that no high resistance can be obtained against wearing of the indicia provided on the upper surface of the key top plate and that the greasy material excreted from the finger tip of the operator permeates the silicone rubber layer of the key top plate to reach and infiltrate the movable contact point resulting in an increase in the electric resistivity thereof. Several remedial means described below are heretofore undertaken to solve these problems.
Firstly, the top surface of the key top plate of a push button switch covering member provided with an indicia is coated with a silicone resin-based coating composition as is disclosed in Japanese Patent Kokai 62-268025. Secondly, the top surface of the key top plate of a push button switch covering member provided with an indicia is first subjected to a treatment with corona discharge followed by application of an amino silane-based coupling agent as a primer and coating with a urethane resin-based coating composition.
The above mentioned first way is in fact effective in imparting the upper surface of the key top with a rigid and dry touch feeling to the operator's finger tip but almost no improvement can be obtained thereby in respect of the wearing resistance of the indicia by the repetition of key-pushing operations. The second way mentioned above is also effective in imparting the upper surface of the key top with a rigid and dry touch feeling to the operator's finger tip and, in addition, is also effective in the improvement of the wearing resistance of the indicia and prevention of infiltration of a greasy material through the key top plate to reach the movable contact point below the key top plate but a problem left in this way is that the coating layer of the urethane resin-based coating composition is susceptible to yellowing in the lapse of time.